For mooring, loading and unloading of a vessel at shallow waters, for example a vessel of the FPSO-type, it has proved difficult to arrange anchor chains with sufficient slack such that a suitable resilience is achieved in the mooring system. At too low resilience in the mooring system, sudden vessel movements may result in too large forces. Another crucial factor regarding mooring is to achieve a position restoring force in the mooring, such that the vessel as moored can lay in a stable position during loading and unloading without occurrence of too strong forces.
Regarding conventional single anchoring systems with a floating buoy connected with a chain or a connection arm to a foundation structure fastened to the seabed, it has proved that the position restoring forces are insufficient.
Completely submerged systems for mooring and transfer of load and signals are possible, but significant problems appear in that critical components are submerged in the sea. This shortens the lifetime significantly and results in severe problems with respect to repair and maintenance.
In view of the above, mooring, loading and unloading at shallow waters are preferably undertaken by use of a mooring tower that extends from the seabed and up to above sea level, with a sufficiently resilient anchoring of the vessel to the top of the mooring tower.
Patent publication U.S. Pat. No. 4,516,942 contains description of a mooring tower that extends from the anchoring at the seabed to over the sea level, where the tower is provided with a triaxial rotatable yoke in the top in the mooring tower, for mooring and transfer of fluid and signals. The yoke's fastening in the top of the mooring tower results in that the mooring tower is subject to a very large momentum from the mooring forces, and the dimensioning of the mooring tower is therefore very powerful. Also the vessel that is to be moored to the mooring tower according to U.S. Pat. No. 4,516,942 must have a powerful dimensioning in order to handle the forces involved, in particular since the main structure of the vessel has to take up forces from the yoke from above deck level. The volume of the mooring tower and the yoke above the water line and down to the deepest draught for vessels in the waters is very significant, resulting in risk for collision and heavy impact from ice, current, waves and wind. As apparent from the above a number of disadvantages are associated with the mooring tower according to U.S. Pat. No. 4,516,942.
The objective of the present invention is to provide an arrangement for mooring without the disadvantages associated with the mooring tower according to patent U.S. Pat. No. 4,516,942 and other relevant prior art, as referred to above.